Conclusion

Abstract

Since a racemization reaction of a chiral alkyl group bonded to a cobalt atom in a cobaloxime crystal on exposure to visible light was found in 1977, a variety of photoreactions of the cobaloxime complexes were observed with keeping the single crystal form. We call them “crystalline-state reactions” discriminating the usual solid-state reactions. The most important merit of the crystalline-state reactions is that we can directly “observe” the structural change occurring in a crystal by X-ray crystal structure analysis. By virtue of crystalline-state reactions, the mechanisms of racemic-to-chiral transformation of the 1-cyanoethyl group only by photo-irradiation, and the chirality inversion process of the 1-(ethoxycarbonyl)ethyl group, an unstable intermediate structure of the bulky R-1,2-bis(methoxycarbonyl)ethyl group can be made clear in the observation of the structural changes. Of course, the metastable structures of carbenes, nitrenes, trans-keto forms of salicylideneanilines, lophyl radicals, and the unstable excited structures can be observed in the crystalline-state reactions.